Filtration

A number defined as the π-Value allows the designer of filter paper to optimize its perm-pore relationships.  It is defined as:

The pore size, measured in microns, can either be the maximum or mean flow pore size. The lower the π-Value, the better is the perm-pore relationship

The π-Value is used to classify various pulp fiber sources for filter media. For example, the π-Values of hardwoods are lower (and better) than softwoods.  Even within a classification, differences are demonstrated.  Within the hardwood category, Eucalyptus pulps are shown to have better π-Values than Northern or Southern hardwood kraft pulps.  Among other pulps that are categorized by the π-Value are Southern and Northern softwood krafts, high bulk wood pulps (mercerized, flash dried and crosslinked), cotton linters, and Esparto grass.

The π-Value of a cellulose medium is closely related to its fundamental fiber properties, particularly fiber diameter and fiber mass.  It is also related to its structure, including thickness and porosity. It is derived by combining Darcy’s Law for porous media and the Hagen-Poiseuille Equation for flow through a cylinder.  

A strong correlation between the π-Value and fiber coarseness is demonstrated. The π-Value is used to classify various pulp fiber sources for filter media. For example, the π-Values of hardwoods are lower (and better) than softwoods.  Even within a classification, differences are demonstrated.  Within the hardwood category, Eucalyptus pulps are shown to have better π-Values than Northern or Southern hardwood kraft pulps.  Among other pulps that are categorized by the π-Value are high bulk wood pulps (mercerized, flash dried and crosslinked), cotton linters, and Esparto grass.

A log weight averaging formula is shown to be very effective in predicting the π-Values of furnish combinations based on the known π-Values of the component pulps.  Predicted π-Values were verified by actual experimentation.  The procedure allows the filter media designer to choose those furnish components that optimize the Perm-Pore properties of the media without actually having to do the experiments.

The field of industrial filtration has changed significantly over the past many years. Several factors have influenced this evolution, such as a need for increased filtration capacity within the same or smaller area, new stricter emission regulations for fine dust particulate, and increased use of dust collectors for product collection with simultaneous control of pollution. Use of industrial bag filters operated in the principle of pulse-jet filtration has got rapid surge as it prove to be most efficient and versatile. These bag filters, which are predominantly surface filters, are mostly made from needle punched nonwoven fabric. Primary factors which determine the selection of a fabric for a particular application are state of aerosol medium (particle size distribution, aerosol velocity, thermal and chemical condition, static charge on the particles, abrasive particles, moisture in the gas stream etc.), filtration requirement (particle capturing efficiency, pressure drop and cleaning) and equipment consideration. Despite the wide applications of nonwoven fabric, the functioning of fabric filters is poorly understood in a pulse-jet filtration process.In the said process, pulse jet cleaning is a technique whereby a short, periodic, high pressure burst of air is fired into the clean side of the fabric. The particles are dislodged and the pressure drop falls to an acceptable level. Pulse jet filtration can meet the stringent particulate emission limits regardless of variation in the operating conditions. Due to the complex characteristics inherent in the filtration and cleaning process of pulse jet bag houses, the proper understanding of the role of fabric construction and aerosol charactersitics on the filtration process are very important.
 
Filtration behaviour based on pulse jet cleaning was studied on a fabricated filtration apparatus. The effect of area density, needling density of nonwoven fabric, and particle size and face velocity on filtration performance of needle-punched polyester filter fabric are studied based on factorial 2 level design.  The table below indicates the level of factors.

1. Quartz powder particles having mean volume diameter of 18.97 cm and mean surface area diameter of 10.87 cm.
2. Sea sand particles having mean volume diameter 105.02 cm and mean surface area diameter of 42.66 cm.

The studies indicate that both filtration efficiency and pressure drop across the fabric at the first and tenth cycles are higher for the fabric having higher area density. Needle density, particle size and face velocity do not affect filtration efficiency significantly. However, pressure drop is affected with a change in face velocity, area density and particle size.

To evaluate the efficiency by particle size, particle counters working with the scattered light principle are used.  This is the technical standard already e.g. in the EN779, Ashrae 52.2 (General Ventilation Filters), EN1822 (High Efficiency Particulate Air Filters) and ISO11155-1 (Cabin Air Filters) and will be introduced into the new standards for engine air intake filters ISO/CD 19713.

Especially measurements in high concentrations can be made with highest safety for the customer due to the new coincidence detection of the white-light-aerosol-spectrometer welas.®  By a short and easy change of the particle sensors it is as well possible with this system to measure initial fractional efficiencies in very low concentrations of the test aerosol in order to avoid the loading of the filter during test procedure.  Additionally, it is possible to measure with the new sensor welas® 2100S unambiguously the particle size and quantity in very high concentrations up to 1000 mg/m3 (ISO-Fine test powder) without dilution systems to evaluate fractional efficiencies during the loading steps.

Since the aerosol concentration in the different processes may change rapidly, a quick particle characterization upstream and downstream of the filter is required.

Therefore, Palas® has developed the new aerosol spectrometer welas®-3000 with two integrated sensors for upstream and downstream which allow carrying out quasi simultaneous particle size and particle concentration measurements.  The sensors may be equipped with different sizes of measuring volumes in order to get optimized measuring results with regard to different concentrations in a short time.  Since the electrical parts-such as the lamp source and optical receiving unit of the scattered light-are the same, the device parameters of the two sensors-such as classification accuracy, resolution and counting efficiency-are the same as well.

To minimize sampling losses the welas®-3000 system works with two sensors installed directly at the test duct.  These two sensors are connected via optical fibres and a new opto-mechanical switch to only one electrical control unit.

During this presentation we will show on the example of a modified test system for filter media, which is ale to perform measurements of filter media in accordance with ISO/CD 19713 (Engine Intake Filters), Ashrae 52.2 and EN779 (General Ventilation Filters), how such a test system must be evaluated concerning the performance requirements of the mentioned standards.

Nanofiber media are useful in many medical, industrial, energy, electronic and filtration applications due to their fine pore structure and high surface area.  Electrospinning is currently the best available technology for producing nano scale polymeric fibers.  This process produces nanofibers with diameters from 5 nm to 5 um by applying a high voltage source to a polymer solution or melt.  As a jet of the solution is ejected from a nozzle it is attracted to a grounded source.

The basic electrospinning process has been around for over seventy years.  Since that time there have been many variations on the original electrospinning process.    Over the past ten years, there has been a significant amount of research into the possible fibers, polymers and applications that can be produced by electrospinning.  Recent advances in the technology have resulted in more cost effective non-woven composites and webs with nano sized fiber diameters. 

This paper evaluate the different types of electrospinning processes with respect to polymers that can be used, products that can be fabricated, performance that can be achieved and markets where they can be used.

The degree of cure of phenolic impregnated filter media is important to the process conditions needed to make high quality filter elements. Historically, phenolic media is pleated then cured. New media systems being introduced in North America are just the opposite, they come to the filter manufacturer already cured – pleating is done after the cure. As a result, understanding how cure level relates to media processing is important. This paper explores various methods used to determine cure while examining the parameters related to cure that influence media processing.